Say cruising at FL310 and on a 270 heading with autopilot on. If pilot decides to descend to say, FL180 on a 180 heading, is simply changing the altitude and heading numbers on the autopilot control panel enough for the autopilot to get going with the new commands, or after changing the altitude and heading request, there is some sort of button that commands the autopilot to execute the new commands? And if so, where is that switch (or button) and what is it called?
It is complicated (-;
First thing first, autopilot systems come with a variety of forms and complexity. The autopilot on a Cessna 172 or Piper Cherokee is going to be very different than the one on a Airbus or Boeing. Different manufacturer will also design their autopilot interface differently; while there are certainly similarities, there is no set standard in the aviation industry for autopilot interface and behavior.
(...) is simply changing the altitude and heading numbers, or (...) there is some sort of button that commands the autopilot to execute the new commands?
Here I will discuss using the Boeing 777 autopilot, which I am familiar with. On the autopilot panel, there is a heading window, which displays a 3-digit number. Directly below the window is a knob, turning the knob counterclockwise will decrease the heading, while turning clockwise will increase it. The knob can also be pushed.
The heading section of the Model Control Panel on a Boeing 777. The knob is a 5-in-1 control: the outer ring can be rotated, the knob can be rotated, and the center of the knob is a push button.
If the autopilot is in "HDL SEL" mode, it will fly the heading specified in the window. In this mode, if the pilot rotates the knob to adjust the heading, the new heading will take effect immediately. However, if it is in "HDL HOLD" mode, turning the knob will adjust the number, but it will have no effect - the plane will hold the current heading. Pushing the knob will change the mode from "HDL HOLD" to "HDL SEL", and the plane will then turn towards the selected heading.
A similar logic exists when climbing using the "VNAV" mode. Let's say the plane is initially cleared to 5000 feet after takeoff. The pilot set the altitude window to 5000 prior to the takeoff roll. Once autopilot is engaged, it will climb to 5000 feet, after which it will hold that altitude. At any point, the altitude knob can be rotated to set a new altitude, say 7000 feet, however it will not take effect - the plane will still climb to 5000 feet, then stop the climb there. To climb to a higher altitude, the pilots have to push the knob, after which the plane will initiate a new climb from 5000 feet to 7000 feet. This is because ATC often clear planes to "step climb", i.e. climbing then holding at certain altitudes until reaching the cruise altitude. The autopilot function is designed around that.
where is that switch (or button)?
It depends on the model. Other manufacturers call them differently, but the logic is similar. Also, advanced autopilots have a lot of functions; the ones I discussed are just selected examples about heading and altitude selections. Pilots can also "arm" an autopilot mode, i.e. mark it so that it will become engaged when certain conditions meet. The specifics of that are too long to describe in this post.
Like I said, it gets complicated.
The terms you are looking for are TARGET, ARM, ACTIVE, CAPTURE, and HOLD.
The pilot selects FL 180, that's a new TARGET. Then they choose one of the different ways to descend*, and they ARM that one. If the A/P is allowed to**, the armed mode becomes ACTIVE. As the plane is about to reach FL 180, the A/P will change to CAPTURE, once the FL is captured, the A/P will HOLD the new flight level.
Simpler autopilots do not have all those features, and require more button pressing. Shown below is the mode control panel (MCP) for the different A/P and flight director functions on the Boeing 787, it is found on the glareshield, right above the main screens in front of the crew.
* The various ways to descend:
- Maintain a vertical speed (V/S)
- Maintain a forward speed with the thrust idle (FLCH or OPEN)
- Maintain a flight-path angle (FPA)
- Follow a varying programmed vertical path / profile (VNAV or PROF)
There are fewer modes to turn or follow a lateral path. And an approach or ILS mode combines both vertical and lateral guidance.
** The A/P may not be allowed to select a heading when the ILS is captured, for example, so turning the A/P OFF then ON is needed to reset it to allow a mode change.
It varies between AFCS units, but in general most have different steering and vertical selection modes. Steering modes in general allow the AFCS to track either a desired heading or a course generated by terrestrial or GPS navaids. Vertical selection modes in general allow the pilot to enter an desired altitude and various types of climb methods ie constant vertical speeds, constant airspeed climbs, vertical descent planning, etc.
In the scenario you just mentioned a pilot might set his AFCS to HDG mode, then select 180 on your heading bug knob. ALT mode would the be selected, dial in 18000 on your altitude bug, then select V/S mode and select -1000 to set a descent rate. The autopilot can then fly the aircraft on a heading of 180°, and descend the aircraft from FL310 to FL180 at 1000 feet/min.
Heading change is much less critical than climb or descent. So most of the autopilots will start turn immidiatly. Climb or descent is more complex. You can run out speed and stall in climb, over speed in descent or hit the ground. Also all turnings are done with preset bank. For climb or descent autopilot has to know how to do that. Keep vertical speed or forward speed or specific angle. So more buttons to push. Also this also helps with additional monitoring of the selected altitude. Pilots has to monitor this more actively because you can soon end at the wrong speed. In turn you can loose several knots but it most cases no any problems at all. Otherwise if you set wrong vertical speed or forget to add throttle in climb you can stall very soon.